The present invention relates to an apparatus for treating wood chips to enhance liquor penetration in subsequent pulping operations. More particularly, the present invention relates to destructuring apparatus in which chips are passed between closely spaced rolls whose surfaces are aggressively contoured for causing chips to be cracked by compressive forces.
In the production of paper from wood fibers, the wood fibers must be freed from the raw wood. In one widely used method, this is accomplished by cooking the wood fibers in a solution until the lignin which holds the fibers together is dissolved. It is desirable to minimize damage to fibers from over cooking. If wood chips of non-uniform thickness are sent to the digester, some chips will be over cooked before thicker chips are completely digested. In order to achieve rapid and uniform digestion by the cooking liquor, the wood, after it has been debarked, is passed through a chipper which reduces the raw wood to chips on the order of one inch to four inches long. The chipper tends to produce a large percentage of over-thick chips which, after separation on a bar screen, must normally be reprocessed through a slicer to reduce them to the desired thickness. This reprocessing through a slicer has the undesirable effect of creating excessive sawdust and pins. The production of sawdust and splinters reduces the overall yield of fibers from a given amount of raw wood. Because the cost of the raw wood is a major contributor to the cost of paper produced, re-slicing the oversized chips incurs a considerable cost.
An alternative to re-slicing over-thick wood chips is a process known as "destructuring" the chips. The chips are fed through opposed rollers which have aggressively contoured surfaces, for example surfaces formed with an array of pyramid-shaped projections. Compressing the chips as they pass through the nip of the rollers produces longitudinal fractures along the grain of the wood. The cracks induced in the chips allow the cooking liquor to penetrate the interior of the chip, thus effectively reducing the chip's thickness. U.S. Pat. Nos. 4,953,795 and 5,385,309, which are hereby incorporated herein by reference, teach the construction of rolls which destructure the wood chips by cracking them preferentially in the direction of the grain.
Improvements in chip destructuring technology which reduce acquisition costs and simplify maintenance and installation would further improve the advantages provided by chip destructuring machines.